The necessity to bond one inorganic insulator, typically a glass body, to another inorganic insulator body, such that the bodies are spaced apart, with a predetermined spacing therebetween, arises frequently in science and technology. For instance, it arises in the manufacture of liquid crystal (LC) display cells.
The prior art has solved the problem by use of adhesive, e.g., epoxy, with spacer bodies suspended therein, together with, typically, placement of spacers in adhesive-free regions between the bonding surfaces. The spacers typically are glass fibers, powder particles, styrene spheres, or other small bodies of relatively closely controlled dimensions. See, for instance, U.S. Pat. No. 4,362,771. With such techniques, uniformity and/or accuracy of cell thickness is frequently difficult to achieve.
It has been known for some time that a strong bond can be formed between a metal (including a semiconductor) and a glass (or other appropriate inorganic insulator) by application of an appropriate voltage across the glass/metal interface, while maintaining the parts at elevated temperature, e.g., 300.degree.-600.degree. C. See G. Wallis and D. I. Pomerantz, Journal of Applied Physics, Vol. 40(10), pp. 3946-3949 (1969), and U.S. Pat. No. 3,397,278, both incorporated herein by reference. Such field-assisted bonding has, inter alia, been employed to adhere the semiconductor substrate of a solar cell to glass pads on a support structure. See U.S. Pat. No. 4,097,309.
Some LC display cells require, in order to function properly, maintenance of plate spacing uniformity to within very narrow limits. For instance, in bistable cells of the type disclosed in U.S. Pat. No. 4,239,345, a typical plate spacing is about 10 .mu.m,.+-.1%. Such accuracy is difficult to achieve with prior art methods. In cells of the type disclosed by clark et al (U.S. Pat. No. 4,367,924) typical spacing is about 1 .mu.m. Again, cells with such small spacings are difficult to fabricate by prior art methods.
Since bistable LC cells and other articles that require bonding two inorganic insulator bodies, with closely controlled spacing between the bonded surfaces, are of considerable technological and scientific importance, a simple method for accomplishing such bonding, over a wide range of spacings, would be of great interest. This application discloses such a method.